The invention relates to the delivery of data services to customer premises, and in particular to a Symmetrical Digital Subscriber Line (SDSL) system adapted to be spectrally compatible with an Asymmetric Digital Subscriber Line (ADSL) standard system.
SDSL is a high-speed data service that can be provided over a standard telephone line (twisted copper wire pair) referred to as a xe2x80x9csubscriber loopxe2x80x9d. Unlike Asynchronous Digital Subscriber Line (ADSL) which provides fast downstream data transport to a subscriber""s premises and slow upstream data transport to the service provider, SDSL provides data transport at the same speed in each direction. Many World Wide Web-based applications that benefit from SDSL are now being developed and the demand for the service is growing.
Traditional SDSL systems achieve symmetry in the data rates by using the same modulation technique, using the same signal bandwidth and transmitting at the same output power from each end of the communication medium. The use of that type of signals creates Near End CrossTalk (NEXT) between co-located transmitters and receivers and limits the service reach, i.e. the length of the subscriber loop over which the service is supported is limited.
Various techniques have been proposed for extending the data transmission reach of SDSL systems. These techniques include the use of coding gain to permit operation at very low SNR, and the use of higher Pulse Amplitude Modulation indexes to reduce the bandwidth of the transmitted signal in order to operate in a frequency band where Near End CrossTalk (NEXT) interference is not severe. Each of these techniques provide only a small incremental benefit.
Another proposed technique for extending SDSL reach is the use of signal repeaters. This however creates additional costs and operational problems in the deployment, the powering and the maintenance of such repeaters. The use of signal repeaters also creates a spectral compatibility problem by introducing high signal levels into the cable at a location where the signal levels are normally low. This problem can be solved by installing a signal repeater on all the pairs carrying a digital data signal and sharing the same cable, but is very difficult to manage when the cable is shared by multiple service providers. For those reasons, it is preferable to deploy digital data services without using signal repeaters.
Data services are also offered using Asymmetric Digital Subscriber Line (ADSL) service over copper twisted pairs. ADSL systems are adapted to reduce NEXT interference from other ADSL equipment using a Frequency Division Multiplexing (FDM) technique to transmit and receive signals. Data transmission over ADSL systems is regulated under the ANSI T1.413 standard which defines a spectral density for the upstream and downstream frequency bands.
When ADSL data service is provided along with SDSL service from the same central location, via the same cable and over adjacent twisted copper pairs, NEXT interference cannot be reduced in either system using signal filters because the interference is within the input frequency band of each receiver. Managing cross-system interference generates high operating costs to service providers, which is ultimately passed on to subscribers.
Solutions that have been proposed to mitigate the interference of SDSL and ADSL services deployed over the same cable include deployment rules that limit the number of SDSL and/or ADSL subscriber loops per cable or cable binder, or require binder group segregation between SDSL and ADSL systems. This solution introduces additional operating costs when both services are provided by a single service provider, and does not support a de-regulated environment in which multiple service providers provide data services from the same central location.
With the rapid deployment of consumer ADSL services, the demand for SDSL services is expected to increase. SDSL services are important, for example, in web site provisioning. Satisfying the demand for SDSL services should not be done at the expense of degrading the performance of ADSL services.
There is therefore a need for apparatus and methods to provide a long data transmission reach for the delivery of SDSL services while reducing NEXT interference. Reduction of NEXT interference between SDSL and ADSL services deployed over the same cable is also desirable.
It is an object of the invention to enable the deployment of SDSL and ADSL services from the same copper plant by reducing electromagnetic interference between SDSL and ADSL services deployed in the same binder group.
It is a further object of the invention to enable the deployment of SDSL and ADSL services on adjacent copper pairs in the same binder group.
In accordance with one aspect of the invention, there is provided a method of transmitting Symmetric Digital Subscriber Line (SDSL) signals over twisted pair copper wires in a cable that interconnects a central location having a plurality of Digital Transmission Units (DTU-C) with a plurality of subscriber premises respectively having at least one Digital Transmission Units (DTU-R), the method comprises a first step of dividing a transmit power spectrum into an upstream frequency band and a downstream frequency band. Signals from the DTU-R are transmitted using the upstream frequency band, and signals from the DTU-C are transmitted using the downstream frequency band.
In accordance with another aspect of the invention, there is provided an apparatus for transmitting Symmetric Digital Subscriber Line (SDSL) signals over twisted pair copper wires in a cable that interconnects a central location having a plurality of Digital Transmission Units (DTU-C) with a plurality of subscriber premises respectively having at least one Digital Transmission Units (DTU-R). The apparatus comprises a Digital Transmission Unit (DTU-R) for transmitting signals from the customer""s premises using an upstream frequency band, and a Digital Transmission Unit (DTU-C) for transmitting signals from the central location using a downstream frequency band. The upstream and the downstream frequency bands are discrete frequency bands that do not overlap.
In accordance with yet a further aspect of the invention, there is provided a method of providing Asynchronous Digital Subscriber Line (ADSL) and Symmetric Digital Subscriber Line (SDSL) services from a central location, comprising a step of offering the SDSL service using Digital Transmission Units (DTU) at the central location (DTU-C) that generate transmit signals in a first frequency band, and Digital Transmission Units (DTU) at customer premises (DTU-R) that generate transmit signals in a second frequency band that does not overlap the first frequency band. The first and second frequency bands are spectrally compatible with corresponding frequency bands used by the ADSL service.
In accordance with yet another aspect of the invention, there is provided a method of providing extended-reach Symmetric Digital Subscriber Line (SDSL) service, comprising using Digital Transmission Units (DTU) at a central location (DTU-C) that generate transmit signals in a first frequency band, and Digital Transmission Units (DTU) at customer"" premises (DTU-R) that generate transmit signals in a second frequency band that does not overlap the first frequency band.
The invention thereby provides methods that enable widespread deployment of SDSL at a reduced cost while improving the service range by increasing the SDSL service reach.